1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic system copying machine, a facsimile, or a printer, and to a unit detachably attachable to a main body of an image forming apparatus.
2. Related Background Art
In an image forming apparatus adopting an electrophotographic system, generally there is known such an image forming apparatus that, for the purpose of preventing an image density from largely varying according to various conditions, such as a change of environment of the image forming apparatus or the number of prints, a developer image for density detection (hereinafter referred to as "patch") is formed on a photosensitive drum as a latent image bearing body whenever image formation for a predetermined number of paper sheets is carried out, the developer density of the patch is detected by an optical sensor, etc., and the detected developer density is fed back to control image formation conditions, such as a developing bias of a development processing condition, so that an image density control is carried out to keep the image density at a predetermined density.
In the foregoing image density control, when the image density control is started, an image density control circuit provided as adjusting means in the image forming apparatus causes a pattern generating circuit to generate an image signal expressing a patch for density detection, and based on this signal, latent images for n patches P1 to Pn are formed along the rotation direction on the photosensitive drum. Next, the latent images are developed by a developing device as developing means. At this time, a high voltage control circuit changes a developing bias (VDC) for each of the patches so that the patches P1 to Pn are developed with developing biases V1 to Vn, respectively. Densities D1 to Dn of the patches P1 to Pn formed on the photosensitive drum are respectively measured by a density sensor.
In the case where the latent images of the patches for density detection are developed by the different developing biases (VDC), the relation (V-D characteristic) between the developing bias (VDC) and the density (O. D.) of the patch becomes as shown in FIG. 4. As is apparent from FIG. 4, the V-D characteristic is composed of parts A and C where the change of the characteristic is small, and a part B where the characteristic is largely changed. This V-D characteristic varies also in accordance with an environment where the image forming apparatus is installed. For example, such a characteristic as shown in FIG. 5 is obtained. In FIG. 5, a characteristic a is the same as that of FIG. 4, a characteristic b is one under a high temperature, high humidity environment, and a characteristic c is one under a low temperature low humidity environment.
As shown in FIG. 4, in the V-D characteristic, the change of the density is unstable in the parts A and C, and the density is stably increased in the part B. Thus, as shown in FIG. 5, with respect to image density control, a control target density DTarget is set in the part B, and the developing biases V1 to Vn are set such that the densities D1 to Dn of the respective patches become D1&lt;D2&lt; . . . &lt;Di&lt;Di+1&lt; . . . &lt;Dn, and the control target density DTarget falls into almost the middle portion of the densities D1 to Dn. The values of the developing biases V1 to Vn are set such that even if the V-D characteristic is slightly changed and the values of the densities D1 to Dn are changed, the control target density DTarget falls within the range of the densities D1 to Dn, and an interval w between the developing bias V1 and the developing bias Vi+1 shown in the drawing is set at about 50 V.
As described above, since the V-D characteristic varies greatly in accordance with the environment, when the values of the developing biases V1 to Vn are fixed, like the characteristic b and the characteristic c shown in FIG. 5, the control target density DTarget deviates from the range of the densities D1 to Dn. Then, the developing biases V1 to Vn are also changed according to each environment so that the control target density DTarget falls almost into the middle portion of the densities D1 to Dn. For example, as shown in FIG. 6, under a high temperature high humidity environment, the developing biases V1 to V4 are used to carry out the image density control.
When the image density control is started, among the developing biases V1 to Vn, ones suitable for the image density control at that time are selected in accordance with an absolute amount of moisture in the apparatus calculated from a temperature and moisture sensor provided in the image forming apparatus. By using the data of the densities D1 to Dn of the respective patches measured by the density sensor and the developing biases V1 to Vn at the formation of the respective patches, a developing bias VTarget optimum for obtaining the control target density DTarget is calculated in the image density control circuit.
A method of calculating the optimum developing bias is such that, among the densities D1 to Dn, an interval in which the control target density DTarget is contained, that is, an interval (i to i+1) where Di.ltoreq.DTarget.ltoreq.Di+1 is established is searched. In the case where such an interval is found, the developing bias VTarget for obtaining the DTarget is calculated using linear interpolation on the basis of the equation 1. EQU VTarget={(Vi+1-Vi)/(Di+1-Di)}.times.(DTarget-Di)+Vi (Equation 1)
The optimum developing bias VTarget is calculated with the above equation.
This developing bias VTarget is held in a memory, and image formation is carried out by using this value until the next image density control is carried out.
However, in such an image forming apparatus, the V-D characteristic varies not only in accordance with an environment where the apparatus is installed, but also in accordance with a driving state of the apparatus. For example, like a characteristic c shown in FIG. 7, an amount of electric charge of a developer is temporarily lowered after a long dormant (sleep) state, so that the V-D characteristic is shifted to a low density side.
As a result, there is a fear that the control target density DTarget will deviate from the range of the densities D1 to Dn and an error will occur. If the V-D characteristic is further shifted through addition of conditions such as deterioration in durability of the developer, the possibility that an error will occur is further increased.
In the case where an error occurs, a process to select a default developing bias previously set as a value of the developing bias VTarget must be carried out. For example, the default developing bias is such a value as an intermediate value between V1 and Vn, V1 if DTarget&lt;D1, or Vn if Dn&lt;DTarget.
In this case, only a minimum image is assured, and an image having a stable density can not be obtained. In order to suppress such a state to the utmost, a method is conceivable that the interval w between the respective developing biases V1 to Vn is widened or the number of patches is increased to widen the range of the developing biases which can be controlled. However, there are problems that an error in linear interpolation may become large in the method of widening the interval between the developing biases, or the amount of consumed developer may become large in the method of increasing the number of patches.
A decrease in the amount of electric charge of a developer after a dormant period and a shift in the V-D curves are temporary, and when the image formation processing is restarted, they are quickly returned to a steady state. Thus, as the amount of electric charge is recovered, the VTarget determined on the basis of the temporary shift of the V-D curves in the image density control immediately after the sleep becomes an unsuitable value, so that it becomes impossible to obtain an image having a stable density. In order to suppress a state to the utmost, such a method is conceivable that an execution interval of image density control is set to be short so that a suitable VTarget following the recovering process of the amount of the electric charge is obtained. However, such a method causes the image density control to be frequently carried out, with the result that developer consumption becomes large, which is a problem.